Soldering of printed circuit boards which are incorporated into household electrical appliances such as televisions and videos is generally performed by an automatic soldering apparatus. An automatic soldering apparatus is equipped with processing units such as a fluxer, a preheater, a wave soldering bath, and a cooler. As a printed circuit board is being transported by a conveyor, it is coated with flux by the fluxer, preheated by the preheater, has solder adhered to it in the wave soldering bath, and is cooled by the cooler to carry out soldering.
All the processing units installed in an automatic soldering apparatus have an effect on the quality of soldering, but a wave soldering bath has the most influence. Namely, depending on the state of spouting of solder in a wave soldering bath, soldering defects may develop or oxides may adhere to printed circuit boards. Although the processing units of an automatic soldering machine can be stably used for long periods, a wave soldering bath has a shorter service life than the other processing units.
A wave soldering bath is equipped with a first discharge nozzle which spouts molten solder in an agitated state and a second discharge nozzle which spouts molten solder in a gentle state. Since molten solder which is spouted from the first discharge nozzle is in an agitated state, it easily penetrates to locations which are difficult for molten solder to reach, such as through holes of printed circuit boards and corners of surface mounted parts placed thereon, and serves to eliminate unsoldered portions. However, because the molten solder is agitated, when it adheres to a printed circuit board, it forms bridges where it adheres between adjoining portions to be soldered, or icicles develop in which solder adheres to the ends of leads in a horn shape. Therefore, the bridges and icicles are rectified by contacting a printed circuit board having these bridges and icicles developed thereon with the gently spouting molten solder from the second discharge nozzle.
Various methods and means have been proposed for agitating the molten solder by the first discharge nozzle. Each of these proposals exhibits an effect so that elimination of unsoldered portions has been achieved to a certain extent. On the other hand, the second discharge nozzle has not been considered to require any special means or to have any problems since it merely produces a gently spouting state. The second discharge nozzle of a conventional wave soldering bath will next be explained. FIG. 4 is a front cross-sectional view of the second discharge nozzle of a conventional wave soldering bath, FIG. 5 is a partially cutaway perspective view thereof, and FIG. 3 is a side cross-sectional view thereof.
A wave soldering bath 20 has a first discharge nozzle (not illustrated) and a second discharge nozzle 21 installed therein. The wave soldering bath 20 contains molten solder 22. An unillustrated electric heater melts the solder and maintains is molten solder at a predetermined temperature. The second discharge nozzle 21 is constituted by a duct 23, an impeller pump 24, a nozzle opening member 25, and a distributing plate 26.
A pump chamber 27 extending for an arc of roughly ¾ of a circle is formed at one end of the duct 23, and a discharge pump 24 is installed in the pump chamber. As shown in FIGS. 4 and 5, the impeller pump frequently used in conventional discharge nozzles has a large number of blades 28 mounted in a radial direction. A shaft 29 is secured at the center of the top of the impeller pump 24, and the upper end of the shaft is connected to an unillustrated motor. An inlet port 30 is formed in the bottom of the pump chamber 27.
The other end of the duct 23 forms an upwardly extending engaging portion 31. As shown in FIG. 5, the duct of the second discharge nozzle in a conventional wave soldering bath narrows at the outlet of the pump chamber 27 and gradually widens as it extends to the engaging portion 31. The reason why the width of the duct increases in the direction from the outlet of the pump chamber toward the engaging portion is that the impeller pump sweeps molten solder which flows in from the bottom of the pump chamber with the large number of blades, and the outlet of the pump chamber is narrowed in order to increase the pressure inside the pump chamber and discharge molten solder into the duct. However, if the duct width remains narrow, the amount of molten solder which reaches the nozzle opening becomes small. Therefore, the duct is gradually widened between the pump chamber and the engaging portion. As shown in FIG. 6, the width (W3) of the nozzle opening in the second discharge nozzle of a conventional wave soldering bath is the same as the width (W4) of the duct.
The nozzle opening member 25 is sealingly engaged with the engaging portion 31 of the duct, and the distributing plate 26 having a large number of holes 32 bored therein extends across the engaging portion. Molten solder which is discharged from the pump chamber into the duct in an energetic state becomes turbulent, and if it is spouted from the nozzle opening of the second discharge nozzle in this condition, a gently spouting state cannot be obtained. Therefore, the turbulent flow undergoes straightening by the distributing plate. The turbulent flow is straightened when it passes through the large number of holes in the distributing plate, and a gently spouting flow emerges from the nozzle opening. The distributing plate not only performs straightening of turbulence but also has the effect of removing oxides which enter the molten solder as contaminants. In a wave soldering bath, oxides are floating on the surface of the molten solder surrounding the nozzle opening member, and when molten solder which has been spouted from the nozzle opening falls onto the surface of the surrounding molten solder, oxides which have a lower specific gravity are entrained by the falling molten solder and caused to sink downwards in the molten solder. Oxides which sink downwards in this manner are sucked into the impeller pump which energetically sucks molten solder. The oxides enter the duct, are spouted from the nozzle opening together with molten solder, and adhere to printed circuit boards. The distributing plate installed in the upper portion of the duct causes oxides to adhere to the distributing plate and prevents them from moving upwards from the distributing plate.
The nozzle opening member 25 has a front plate 33 installed on the entrance side of a printed circuit board and a rear plate 34 installed on the exit side of a printed circuit board. The front plate makes molten solder spouted from the nozzle opening flow towards the entrance side, thereby remelting and eliminating bridges and icicles formed by the first discharge nozzle. However, if a printed circuit board exits from the second discharge nozzle in this state, the amount of solder adhered to the portions being soldered becomes small. Therefore, the rear plate is adapted to adhere a suitable amount of solder. In the rear plate, molten solder flows in the same direction as the direction of travel of a printed circuit broad at approximately the same speed as the speed of travel of the printed circuit board, so a suitable amount of solder adheres to a printed circuit board contacted by molten solder flowing from the rear plate. As shown in FIG. 6, in the second discharge nozzle of a conventional wave soldering bath, since the width of the duct and the width of the nozzle opening member are the same, molten solder which flows into the duct is spouted upwards in the same state and forms a gently spouting state.
The state of spouting from the second discharge nozzle in a conventional wave soldering bath will be explained. The impeller pump 24 is rotated by driving an unillustrated motor which rotates the shaft 29. Molten solder present between the large number of blades 28 of the impeller pump 24 is then swept by the blades and flows from the pump chamber 27 into the duct 23. At this time, since molten solder is no longer present between the blades 28 of the impeller pump 24, molten solder is energetically sucked into the pump chamber 27 through the inlet port 30 in the bottom of the pump chamber 27. As shown by the arrows in FIG. 4, molten solder which flows into the duct 23 strikes against the front end of the duct with a high flow speed and changes its flow direction upwards. Because the flow speed of molten solder at this time is fast and because it strikes the front end of the duct and changes its flow direction, it becomes turbulent. This turbulent flow undergoes straightening by the large number of holes 32 in the distributing plate 26 extending across the engaging portion 31 of the duct and then spouts upwards through the nozzle opening member 25. An unillustrated printed circuit board which has been soldered by the first discharge nozzle contacts the molten solder spouting from the second discharge nozzle, whereby bridges and icicles formed by the first discharge nozzle are rectified and a suitable amount of solder is adhered before the printed circuit board then moves away from the second discharge nozzle.
The impeller pump shown in FIGS. 4 and 5 is the most common type of pump used in a conventional wave soldering bath, but a spiral pump has also been proposed (Patent Documents 1-5).
Patent Document 1: JP S48-19425 U1
Patent Document 2: JP S48-98520 U1
Patent Document 3: JP S50-148327 U1
Patent Document 4: JP S51-3632 U1
Patent Document 5: JP S62-259665 A1